It is simple a style of Mexican food that comes from southern Texas and northern Mexico. Yankees do tend to get confused about this. Just like the styles of Mexican food you find in New Mexico and Sothern California are different than what you get in most of Mexico the style of Mexican food in south Texas is distinct. The stuff they call TexMex after it left that region is no more authentic TexMex than it is authentic Mexican food. Hell you can hardly find good TexMex here in Dallas.